Microphone



H. SELL MICROPHONE March 21, 1933.

Filed April 6, 1929 factory manner.

reamed Mar. 21, 1933,

UNITED STATES PATENT OFFICE "UT SELL, OF BEBLIN-SIEMENSSTADT, GERMANY, ASSIGNOE 1'0 SIEMENS &

HAJLSKE, AKTIENGESELLSCHAI'T, OF SIEMENSSTADT -NEAB BERLIN, GERMANY, A

mrcnornonn hpplication filed April 6, 1929, Serial No. 353,211, and in Germany April 7, 1928.

5 electrical fluctuations and has the object of removing certain disadvantages inherent to microphones used at the present day.

When high quality of sound transmission is required of a microphone, in particular of 1 a carbon powder mlcrophone, the usual type heretofore used owing to the well known peculiar properties of such microphones, has certain disadvantages owing to which the sound is not transmitted in an entirely satis- These defects 0 are principally due to two characteristic features inherent to carbon microphones. First, its high threshold value i. e. its failure to respond at small sound intensities or amplitudes.

3 Second, the disturbing internal microphone noises'at appreciable current intensities, ow-

ing to the instability of the individual contacts between the carbon granules; To these two principal defects are added the distortions due to its varying sensitiveness at different frequencies and to its general varying sensitiveness at difi'erent times irrespective of the sound frequency.

- The present invention is based on the genao eral recognition that it is impractical in carbon granular microphones to transmit the sound through long stretches of granular material. When the sound is compelled to pass through long stretches of carbon powder or at granulated powder the sound waves will, so

to say get stuck in the long path through the material, owing to the blocking phenomena caused by mechanical adhesion of the minute particles which oscillate relatively to each ao other about any point of equilibrium which happens to develop or exist. Thus it happens that the portions of the granular material encountered later by the sound wave are agitated in a by far more hap-hazard way than. the portions of the granular filling immediately adjacent to the diaphragm, so that at one time the entire filling may be very strongly agitated throughout its entire length and at other times the zones far removed from the so diaphragm may be very little agitated for longer or shorter stretches. In such a case these dead, or only slightly excited zones merel act as a series resistance. Thus, the function of a long granular path does not constitute a uniform function during which all the sound energy is uniformly transmitted to the entire mass of powder and by it trans-' formed into electric energy, but it constitutes a rather unsteady and irregular process. The foregoing thus explains the great dependence of carbon microphones on comparatively 'large sound amplitudes. Of course, the frequency-of necessity-constitutes an important factor, alone for the reason that, owing to the dependance upon a certain amplitude as aforementioned, the many different amplitudes existingat the many frequencies occurring for instance in the transmission of music, are likely to be distorted.

From the above it would appear that it is advisable to make the granular path. as short as possible in-the direction of sound travel in order to avoid the distortions and disadvan tages mentioned hereinbefore. On the other hand, short granular paths ordinarily increase the inconstancy of. the powder so far as its response to varying sound frequencies is concerned, because the material jams between the electrodes and causes a short circuit at which it becomes entirely insensitive. To these viewpoints from which the design of a good microphone must be considered, another point is added. Good reproduction and sound transformation into corresponding electric energy presupposes a suitable diaphragm! Now, it is a feature of my invention to use a very thin diaphragm, for instance made of metal foil which is in contact with the carbon owder throughout its entire area. This resu ts in the mechanical tuning to a very high frequency which insures the independence of the microphone from the frequency over a very wide range. The presumption in such a case is, however, that the granular carbon material contacts with the entire surface of the diaphragm at uniform pressure, because the above-mentioned tuning is a result of the coactionbetween the small diaphragm mass and the elasticity of the carbon granules. In order to roduce in such a case the desired amount constancy and uniformity in operation a comparatively large 6 chamber for the granular material is necessary so as to permit the carbon material to exert a well defined static pressure against the diaphragm alone by its own weight. However, in such an arrangement a granular 10 carbon path of from 1 to 2 millimeter length would cause a congestion of the carbon material in the lower portion of the vertical disc shaped chamber, as in that portion the frictional resistances would exceed the momentum of the individual carbon particles imparted to them by the sound energy. This would cause practically a short circuit between the two electrodes of the microphone and allow a considerable portion of the remainder of the diaphragm to swing by itself without the elastic effect of the carbon powder upon it as aforementioned, so that disturbing emphasized natural frequencies would manifest themselves within the desired range of sound frequencies.

It is the purpose of the present invention to remove these particular difiiculties mentioned above by providing instead of a single smooth-walled chamber for the carbon which affords a carbon contact over the entire diaphragm surface, shallow and relatively long troughs, onl a few millimeters in width but of any suita le form otherwise, these troughs constituting the chamber or chambers for the granular material. The open portion of these troughs, which may be composed also of a single chamber, is according to my invention covered by the afore-mentioned li htest and thinnest possible foil diaphragm 0 suitable material. Corresponding to the long-stretched shape of the chamber or chambers, the diaphragm should be shaped ribbonlike and in consequence of such a form it will have a high natural frequency. The troughs are entirel filled with finely granulated carbon, and t eir width, as above-mentioned, is so small that the powder will lie with a uniform pressure against the diaphragm throughout the ent re area represented by the trough opening. The current supply would pre erably occur at the two ends of the anular path by means of carbon electro es or other suitable material. Such granular carbon paths may either be 5 arranged in a s iral or as straight parallel stretches, electrically connected in parallel or in series, and their electric resistance can thus be made very high. It might be of the order of 100,000 ohms. Such an arrangement affords extremely favorable conditions for the operation and results in great efliciency.

Through practical investigation "and research it was found that the internal noises inherent to carbon owder micro bones have their seat principal y in the neig borhood of the terminal electrodes, because at these points the greatest difference in resistance, in other words, the greatest resistance drop prevails.

According to the novel arrangement described hereinbefore, the eifect of this great difference in resistance is removed by making the resistance of the total carbon path, by suitably dimensioning it as described, very great compared with the difference in resistance existing at the electrodes at which the current passes from a conductor of comparatively low resistance into a conductor of comparatively very great resistance (the granular path).

In considering the effect of a microphone constructed according to the present'invention, one should bear in mind that the reliable operation of such a microphone is due to and represents means values of the function of a very large number of contacts. A long current path according to the present invention would inherently give a very excellent average value, because it is immaterial in such a microphone whether the granular material is acoustically dead at a few places, since it represents the combination of a large number of sound responsive devices of the same or similar construction and in which the temporary inefliciency of one or part of one would not materially affect the total result.

Since it is proposed in accordance with my invention in such a microphone arrangement to use very thin, preferably non-metallic foil, the natural frequency might be placed at a very high value so that it lies outside of for instance the highest musical frequency values, and thatit might still have a good sensitivity even at a frequency of about 10,000.- Using a non-metallic foil it might be valuable to make the bottom of the trough.

of a conductive material preferable in the form of a carbon lining in order to employ it as electrode.

One of the great advantages of my improved microphone is its comparatively great sensitiveness and its excellent adaptability for operation with amplifiers. On account of its high internal resistance it may preferably be connected without atransformer directly with an amplifier in a resistance coupled circuit. Such a connection which is particularly advantageous on account of its independence of frequency, cannot be made in practice with the ordinary type of microphones heretofore used on account of the unfavorable coupling conditions which would exist in such a case, since a large number of amplification stages would first be required before such'a connection could be made. In contradistinction the microphone, 1 according to the present invention, has a high degree of sensitiveness on account of its high inner resistance. Under ordinary circumstances, a

three-stage amplifier is sufiicient for my novel microphone.

When using my microphone with amplifiers the microphone current should preferably be derived from the plate battery of the tube amplifiers. According to the lengths anddimensions and the number of powder troughs connected in series, a smaller or larger voltage, say between 20 and 500 volts is required. It is also advisable to design the microphone so that the full anode potential of the amplifier can be applied. The current consumption, of course, also depends upon the circuit arrangement of the troughs and their dimensions. For instance, in a standard type of microphone constructed according to my invention, the current consumption would amount to about from 0.5 to 1 milliamps, at a potential of 200 volts.

A further advantage of the novel micro phone is that it will stand a considerable current load, because the cooling conditions are extremely favorable on account of the large surfaces and the small depths of the powder troughs, and in view of the very thin diaphragm permissible in such a microphone. For instance loud speakers can be operated already with two microphone amplifier stages at loads of'the microphone at which it is still free from internal microphone noises.

' My invention is illustrated in the accom pa'nying drawing in which Figure 1 arrangement of a plurality of powder troughs but omitting the diaphragm,

Figure 2 shows a transverse section thereof on the line 11-11 of Figure 1, showing also the diaphragm in section,

Figure 3 shows a transverse section similar to that shown in Figure 2 in which a corrugated diaphragm isused,

Figure 4. shows a circuit diagram of the micfirophone connected with a multi-stage amli er, P Figure 5 shows a face view of a microphone employing the spiral arrangement of the troughs,

Figure 6 shows a tranverse section through the microphone similar to Figure 2 its trough having a corrugated bottom.

Figure 7 shows in plan view a microphone the diaphragm of which has transverse ribs,

Figure 8 shows a transverse section through a transverse current microphone whose trough bottom comprises a corrugated carbon plate and whose diaphragm is of insulating material and Figure 9 shows a prism shaped microphone body with two oppositely arranged troughs, their powder paths being connected in parallel by carbon pins.

Referring to Figures 1 and 2, 1 represents the body of the microphone in which are provided a plurality of troughs 2'which contain the granulated carbon. This body 1 is, of

shows in plan view the general course, of insulating material. Each trough contains at its ends the electrodes 33, which protrude into the trough and through which width. They are covered j ointly by the diaphragm 4 of any suitable material and of the character described hereinbefore. The diaphragm 4 is firmly pasted on the surface of the insulating body so that only the portions above the troughs as shown in Figure 2can freely oscillate as long ribbons. The same result will be attained if the troughs are covered by individual strips pasted on to the surface of the body close to the trough'rims. The carbon granules 5 are thus entirely enclosed in the troughs since the diaphragm entirely closes the troughs at their open sides These troughs may be connected electrically in series with each other or in parallel or series parallel as the conditions may require, this being entirely within the province of anyone skilled in this art, it has not been shown in detail.

I Figure 3 shows in cross section'an insulating body 1 similar to that shown in Figures 1 and 2, but which is covered by a corrugated diaphragm 6, so that the top of the trough assumes the form shown in that figure. Of course, any other form of corrugation may be used, as will be described later on.

Figure 4 shows the installation of such a microphone having only one trough 2, in a repeater circuit. The microphone respectively its carbon granular path 2 is with one of its electrodes 3 connected to the battery 7. The other electrode 3 is connected across a resistance 8 to the other pole of the battery 7 in order to generate a drop of potential. The microphone circuit is connected directl with the circuit of a three-stage amplifier, t e two circuits being separated by condensers 9 to prevent the passage of direct current. The amplifier comprises the valves 10 and 11 as well as a terminal valve 12 coupled to each other by resistances 13, 14, the terminal valve bein connected with a telephone or loudspea er 15. A single pole switch 16 serves for switching-in the filament heating current. A connection such as aforedescribed is possible on account of the high resistance and sensitivity of the novel microphone which requires only a three-stage amplification.

Referring to Figure 5 the insulating body 1 of the microphone is disc shaped and its troughs 2 are spiral-like arranged; the inner ends of the spiral troughs are connected by a carbon pin 17 whilst their outer ends are connected to terminals 3. The diaphragm has been omitted in the drawing.

Figure 6 shows in cross-section throu h the microphone body 1 the individual troug s 2, shown in Figs. 1 and 2, combined in a single large chamber. In this case the bottom of chamber 20 is corrugated by longitudinal ridges 18 in contradistinction to Fig. 3 in which the foil diaphragm is corrugated. In Fig. 6, like in Fig. 3, by these means congestion of the carbon powder at the lower portion of the chamber is avoided.

In Figure 7 transverse corrugation is employed as shown at 6', these corrugations be-- ing stamped into the diaphragm foil. This transverse corrugation is employed in case,

to use two kinds of corrugations arranged at right angles, for instance one kind being provided in'the diaphragm and the other at the bottom of the chamber.

Another construction of the microphone is shown in Figure 8. The insulating body is formed by a narrow frame 19 of insulating material the lower opening of which is closed by a corrugated body 21 consisting of carbon to be used as terminal electrode, the upper frame side being closed by the diaphragm 4, the same as shown in Fig. 6, the diaphragm 4 being of insulating material (mica). For other purposes, if a low resistance of the microphone is desired a metal foil diaphragm may be used. The trough 22, formed by the frame 19 and its carbon bottom 21, is filled with the carbon powder.

A very suitable construction of the microphone is shown in Figure 9. The insulating body is a prismatic bar 23. On opposite sides of it troughs 2 are arranged which are connected in parallel by pins 24. These pins are connected at the same time to carbon terminals 25 which are dovetailed into the insulating body. To more clearly show the internal nal electrodes andthe way in which the latter are disposed in the troughs.

Aside from varying the kind of material used, the diaphragm may have various forms. It might for instance be provided to advantage with longitudinal or transverse ribs or other indentations which impart to it a certain stiffness and by which the contact surface with the granular carbon material may be increased. The stifiness is ensured with thin metal foil diaphragms if -the metal used is not too weak.

by other conductive crystalline or granulated material. In contradistinction from the standard microphones used heretofore, in which no practical microphonic effect is obtained with metal powder, in my novel microphone also metal powder may be used in particular of a metal which has high elasticity and great hardness such as tantalum or the like. The required quantity of the granular material is very small as compared with other microphones.

I claim 1. A microphone of the character described, comprising a prismatic body of insulating material having two narrow and shallow troughs provided in opposite sides of said prism, said troughs being filled with granular conductive material, carbon plugs protruding into the ends of said troughs, to direct the microphone current? nly through said material lengthwise of the troughs, and thin foil diaphragms each of them closing one of said troughs and being attached to the trough edges, and contacting with said granulated material, and terminal electrodes in said granulated material attached to the ends of said carbon plugs for conducting current through said material, said diaphragm being sufficiently narrow to raise the natural frequency of the transverse oscillations above the normal musical tone frequency range.

2. A microphone of the character described, comprising a body of insulating material having a narrow and shallow trough provided in its surface whose length is at least five times its width, said trough being filled with granular conductive material and arranged to direct the microphone currents lengthwise through the granular material only, and a thin foil diaphragm covering said trough and being attached adjacent to the trough rim to the surface ofsaid body to oscillate above said trough. and forming with said trough a closed chamber for said granular material, said diaphragm being sufliciently narrow to raise the natural frequency of the transverse oscillations above the normal musical tone frequency range.

3. A microphone of the character described, comprising a body of insulating material having a narrow and shallow trough provided in its surface and having a length at least five times its width, said trough being filled with granular conductive material,

anda thin hand-shaped foil diaphragm fixed adjacent to the trough rim to said insulating body, and closing said trough and contacting with said granulated material as to oscillate freely above said trough, and terminal electrodes in said granular material at the ends of said trough for conducting current only through said material lengthwise of the trough.

4. A microphone of the character described, comprising a body of insulating material having a plurality of narrow and shallow troughs provided in its surface, each trough having a length at least five times its width, said troughs being filled with granular conductive material, and each being arranged to conduct the microphone current lengthwise through the material only, and a thin foil diaphragm pasted on-said insulating body closely adjacent to the trough rims and closing said troughs individually, whereby the individual diaphragm portions are adapted to oscillate above their respective troughs, said diaphragm portions contacting with the granulated material in their respective troughs, the oscillating parts of said diaphragm having a ribbon-like shape simi lar to that of said troughs, said diaphragm being sufficiently narrow to raise the natural frequency of the transverse oscillations above the normal musical tone frequency range.

5. A microphone of the character described, comprising a body of insulating material having a narrow and shallow trough provided in its surface, and having a length at least five times its width, said troughs being filled with granular conductive material and arranged to conduct the microphone currents lengthwise through the material only, and a thin non-conducting band-shaped foil diaphragm pasted at the trough rim to said insulating body and closing said trough and contacting with said granulated material as to oscillate freely above said trough.

6. A microphone of the character described, comprising a body of insulating material having aplurality of narrow and shallow independent parallel troughs provided in its surface, the length of each trough being at least five times its width, said troughs being filled with granular conductive material and terminal electrodes for said granular material at the two ends of each trough for conducting the microphone currents lengthwise through the' material only, and permitting the electric connection of the several troughs in series, in parallel or in seriesparallel with each other,and a thin foil diaphragm of non-conducting material closing said troughs and being attached to the body at all trough rims and contacting with the granular material contained therein, said diaphragm being sufliciently narrow to raise the natural frequency of the transverse oscillations above the normal musical tone froquency range.

7. A microphone of the character described, comprising a body of insulating material having a plurality of narrow and shallow troughs provided in its surface, each trough having a length at least five times its width, said troughs being filled with granular conductive material, and each trough being arranged to conduct the microphone currents lengthwise through the material only, and a thin foil diaphragm pasted on said insulating body at all trough rims, and closing said troughs and contacting with the granulated material therein, said diaphragm being provided with ribs of-suitable configuration for increasing the contact area w th said granulated material and for increasing the stifiness of the diaphragm as well as to avoid the congestion ofsaid granulated material. v

testimony whereof I afix my signature.

HELMUT SELL.

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